The present invention relates, in general, to ophthalmic lenses having photochromic properties, and more particularly to a combination lens fabricated from a glass and plastic laminate to provide a lens having the lightweight characteristics and other known advantages of plastic while at the same time having the superior photochromic properties presently available only in glass lenses.
Photochromic glass is described in U.S. Pat. No. 3,208,860, which defines such glass as having the capability of changing color when exposed to certain types of radiation and then returning to its original color when the radiation is removed. Usually, such glass is sensitive to ultraviolet radiation, although it may also be sensitive to other wavelengths. The reversible optical property imparted to glass in accordance with U.S. Pat. No. 3,208,860 is achieved by incorporating silver halide crystals into the glass. As explained in the patent, the mechanism of photochromic color change is based on the fact that the submicroscopic silver halide crystallites darken under the action of actinic radiation to reduce the optical transmittance of glass. When the source of actinic radiation is removed, the crystallites return to their original color state, restoring the optical transmittance to its original level. This sequence of darkening and fading can be repeated indefinitely without fatigue or loss of photochromic properties.
As pointed out in U.S. Pat. No. 4,168,339, the most extensive application for photochromic glass, to the present time, has been in the fabrication of ophthalmic lenses, both as prescription lenses and as non-prescription sunglasses. Because the incorporation of photochromic materials in such lenses represents compromises made between these characteristics and desired ophthalmic properties, extensive research has continued in order to try to provide a glass demonstrating improved photochromic behavior, while still retaining the other physical attributes demanded in the production of ophthalmic lenses. One problem in particular has been the fact that because the photochromic effect is caused by the absorption of atinic radiation by photochromic particles in the glass itself, the light transmittance of a darkened photochromic specimen is related in part to its thickness. Thus, where other parameters are held constant, a thicker sample of photochromic glass will normally get darker than a specimen of thin dimensions. This creates some problems in ophthalmic lenses where the grinding and polishing required to conform the lens to a desired prescription results in complex variations in thickness throughout the lens, and thus produces variations in the darkness of the lens.
Photochromic glass lenses have met with considerable market acceptability because of the advantages of a color-changeable ophthalmic lens and because glass lenses have a high degree of hardness and scratch resistence, are capable of surviving wide temperature extremes and frequent temperature cycling, and do not change significantly with age. However, glass is very heavy when compared to plastic both in prescription and non-prescription lenses. As a result, plastics have increasingly replaced glass as the material of choice for ophthalmic lenses since they cause less discomfort to the wearer, and this permits the use of lenses of greater area or diameter. The majority of patients now prefer and purchase lightweight plastic prescription lenses, whereas the vast majority of those who still purchase glass lenses do so because they want the photochromic properties now available only with glass.
Other advantages of plastic lenses are that they have a high clarity, can be dyed or tinted easily, machined easily and are relatively stable. Of course, plastic lenses do have certain drawbacks; for example, they do not possess the surface hardness of glass and thus are more susceptible to scratching. Furthermore, they have a very high thermal expansion and exhibit a greater degree of flexibility than glass, making precision optical polishing difficult, and often resulting in optical abberations, i.e., changes in the characteristics of a lens from its center to its pheriphery.
Numerous attempts have been made to produce photochromic articles from plastic materials, as exemplified by U.S. Pat. No. 3,551,344 which discloses a method of incorporating photochromic organic compounds in vinyl-type polymeric resin materials to form a photochromic plastic article. However, products such as this have not been practical for use as ophthalmic lenses because they have succumbed to fatigue of the color-reversible material relatively quickly, sometimes because the photochromic compounds were chemically incompatible with the plastic material and other times because these compounds decompose with exposure to water vapor or oxygen, both of which infuse slowly through plastic materials. The result has been that the photochromic properties that could be incorporated into plastic materials have disappeared in a relatively short time, so that these attempts have not been considered to be successful.
Many attempts have been made to produce a composite lens of glass and plastic wherein a glass layer is either buried within the plastic or is provided on its surface, the plastic being clear and the glass being photochromic. Although glass-plastic laminates have been successfully used in fields such as safety glass and structural glass, where the plastic layer is essentially a coating on a main glass body, the problems in forming glass-plastic composite ophthalmic lenses have been virtually insurmountable. Problems such as delamination, incomplete bonding, stress-induced birefringence, high sensitivity to temperature changes, and the like, have prevented the successful production of a glass-plastic ophthalmic lens that would incorporate all of the positive features of a glass lens, such as hardness, scratch resistance, rigidity, and photochromic properties, as well as the desirable properties of plastic, such as its light weight, its ability to receive dyes and tints easily, and the like. The principal difficulty has been found to be the wide disparity in the thermal expansion of glass with respect to plastic, for glass has a thermal expansion of approximately 5 parts per million per degreee centigrade, whereas the coefficient of expansion for optical plastic is on the order of up to 150 PPM/.degree. C. at higher temperatures (and less than 80 at low temperatures). This difference in coefficient of expansion produces a significant difference in the mechanical expansion of a plastic layer with respect to a glass layer in a laminated lens. Where the glass layer is relatively thin with respect to the plastic layer in order to take advantage of the weight differential of the materials, such a difference in expansion effectively prevents its use as an ophthalmic lens. For example, if a three inch diamter ophthalmic glass/plastic laminate is exposed to boiling water, as would be the case if a tint or a dye were to be applied to a pair of glasses, the difference in the coefficients of expansion of the two materials, would result in a 0.034 inch difference in the diameters of the two laminates, causing the plastic to protrude 0.017 or more inches past the glass lens around its perimeter. Such an expansion is more than adequate to break the adhesion between the lens laminates, with prior art laminations, or else to fracture one of the lens components.
Numerous attempts have been made to overcome the problems that have occurred in the production of a plastic-glass ophthalmic lens, but have been unsuccessful at least in part because the usual lamination procedure, using adhesives such as ultra-violet cured epoxies or any number of other clear adhesives, has been to apply the adhesive to the surface of one of the layers and then press the other layer onto it with sufficient pressure to mate the two layers as uniformly close together as possible. The bonding surfaces of the two layers are normally formed with nearly identical curves so that they fit closely together, with the adhesive being applied therebetween. This close mating, which has been on the order of 0.001 mm, sought to achieve the maximum strength of bonding to prevent delamination. However, in reality it has been found that the plastic lens layer will expand with respect to the glass either beyond the flexing limit of the plastic so that the plastic will fracture, or beyond the flexing limit of the glass so that the glass will fracture, or to the point where the adhesive bond strength will be exceeded so that its adhesion to one or the other of the surfaces will be lost, or to the point where the cohesive strength of the adhesive itself will be exceeded so that the adhesive fractures. Most laminating adhesives which perform well at high temperatures fail at low temperatures, due to the brittleness of the adhesive at such temperatures, or fail at high temperatures because the adhesive becomes soft and flows, leaving stretch blemishes, bubbles, and eventually delamination. Sometimes the adhesive itself has such a widely different thermal expansion characteristic than the glass or plastic layers that the adhesive forces itself to delaminate. Hot water or high humidity is often very damaging to adhesive bonds and is a major source of failures.